The Arizona cities of Flagstaff and Phoenix are relatively close to one another yet have contrasting climates. Why does this difference occur?

Phoenix is located at 1100ft above sea level, and Flagstaff is located 6900ft

5800ft difference, roughly for every 1000ft you go higher the air cools by about 5.4 degrees F.

so 5.8*5.4= 31.3 degrees
flagstaff is typically 31degrees colder than phoenix. And if you actually look at the temperature statistics its true.

Higher altitudes are colder because the air is thinner (there is less of it) and the less air there is the less heat it can hold. This is why even high mountains near the equator get snow, and why flagstaff is so much colder than phoenix,